Subscriber line interface circuits are typically found in the central office exchange of a telecommunications network. A subscriber line interface circuit (SLIC) provides a communications interface between the digital switching network of a central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location remote from the central office exchange.
The analog subscriber line and subscriber equipment form a subscriber loop. The interface requirements of a SLIC result in the need to provide relatively high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are low voltage analog signals on the subscriber loop. Thus the SLIC must detect and transform low voltage analog signals into digital data for transmitting communications received from the subscriber equipment to the digital network. For bi-directional communication, the SLIC must also transform digital data received from the digital network into low voltage analog signals for transmission on the subscriber loop to the subscriber equipment.
The SLIC must be provided with a negative voltage supply sufficient to accommodate the most negative loop voltage while maintaining the SLIC internal circuitry in their normal region of operation. In order to ensure sufficient supply levels, a power supply providing a constant or fixed supply level sufficient to meet or exceed the requirements of all of these states may be provided. However, such solutions invariable result in wasted power for at least some operational states.
One supply level is required when the subscriber equipment is “on hook” and another supply level is required when the subscriber equipment is “off hook”. Yet another supply level is required for “ringing”. A subscriber line interface circuit thus requires different power supply levels depending upon operational state.
One disadvantage of a single fixed power supply architecture is that excess power is generated and must be dissipated as heat or otherwise wasted when a SLIC is not using a power supply level optimized for its particular operational state or for the particular line conditions. For example, the power supply must be capable of supporting the worst-case scenario such as a maximum subscriber line length provided for by specification. In the event the subscriber line is considerably shorter than the maximum expected length, the SLIC will be required to absorb the excess power. The resulting additional thermal load can be problematic for integrated circuits of the SLIC.
One alternative to a single fixed supply is to utilize two fixed supplies. SLIC control circuitry selects between the two fixed supplies based upon operational mode. This approach reduces the amount of excess power wasted at the expense of the operational mode based control circuitry and maintaining two fixed supplies.
More recent architectures utilize switching circuitry (e.g., DC-DC converter) to generate the appropriate supply level (VBAT) from another fixed supply. The switching circuitry can be controlled to track the level needed by the SLIC and provide a variable VBAT. Instead of multiple fixed power supplies to accommodate the different operational states, a single tracking supply varies its output VBAT to meet the SLIC's needs.
The operational states of individual subscriber lines are inherently independent of each other. Each subscriber line may be referred to as a SLIC channel. Each SLIC channel is associated with its own linefeed driver. Providing a single shared fixed supply or providing a shared tracking supply that caters to the neediest channel inherently results in wasted power and heat for devices or channels that do not have the same requirements. Prior art solutions provide separate switching circuitry for each channel or device to reduce the amount of wasted power and heat generation. Each tracking power supply varies its VBAT supply level in accordance with the requirements of its associated channel or device. This tracking power supply architecture is more power efficient than the shared fixed power supply architecture. Given that a tracking power supply is utilized for each channel, however, such an architecture may not be economical to implement—particularly with respect to a large number of channels.